Prop pitch - rule of thumb calcs

[C08]

My outboard came overpropped and max revs is 4500 rather than say 5500 so I am thinking of a smaller pitch prop. Some sources suggest a pitch alteration of 1" will result in an rpm change of 200 other say 300rpm. Any informed views or experiences on this topic?

 

[scottie]

It will depend on a combination of engine HP and size of thumb so both may be appropriate

 

[Chiara’s slave]

My outboard came overpropped and max revs is 4500 rather than say 5500 so I am thinking of a smaller pitch prop. Some sources suggest a pitch alteration of 1" will result in an rpm change of 200 other say 300rpm. Any informed views or experiences on this topic?

I changed ours a year or so ago. I went back to basics, inches per rev, drive ratio, rpm, converted to knots. I daresay there’s an online calculator but it’s not complicated. We are powered by a Honda 15, suited to a small RIB capable of 20kn. It’s not going to push a 2 ton 30ft tri at 20kn, we went from a 10 inch pitch to a 5 inch. We get 8 knots in still conditions, I couldn’t say how many rpm it has changed as we have no rev counter, but it all feels right.

 

[C08]

That is interesting thanks. I want to make a change but need it to be worthwhile. A drop from 9" to 5" pitch (13" prop) should get me to 5300 if the lower or 5700 if the higher figure, both within the 5-6ooorpm range. Somehow against my better judgement I let the Yam dealer convince me this was the right prop for an old heavy catamaram!

 

[bedouin]

That is interesting thanks. I want to make a change but need it to be worthwhile. A drop from 9" to 5" pitch (13" prop) should get me to 5300 if the lower or 5700 if the higher figure, both within the 5-6ooorpm range. Somehow against my better judgement I let the Yam dealer convince me this was the right prop for an old heavy catamaram!

That sounds extreme to me - 5500 on a 5" pitch is much slower than 4500 on a 9" so you will almost certainly lose top speed. You'll probably find yourself running the engine at max revs and still being slower than you were before.

 

[Slowboat35]

There are many prop-calc sites on the internet, what do they say?

 

[Tranona]

The 2-300 rpm for 1" pitch change rule of thumb works for shaft speeds of +/- 1500rpm - that is typical for inboard diesels maxing at +/- 3000 rpm and reduction ratios or 2:1 to 2.6:1.

Outboards I think have a higher shaft speed

 

[Chiara’s slave]

That sounds extreme to me - 5500 on a 5" pitch is much slower than 4500 on a 9" so you will almost certainly lose top speed. You'll probably find yourself running the engine at max revs and still being slower than you were before.

A 9” pitch gives about 20kn at engine rpm of 5500, with an average reduction tatio of 2:1. So a 5” gives about 11.5. Obviously there’s slip to build in, but this is broadly our calculation, we are ‘geared’ by the prop pitch for round about 11kn at peak power. The engine does seem to hit that, pretty much, and the boat will just creep over 8kn. Is there a prob with my arithmetic? In any case, it works. The boat is noticeably faster than it was with the bigger pitch, and it accelerates and stops better.

 

[Chiara’s slave]

Pitch 9” = 0.75ft. 0.75 x rpm/reduction x 60(mins/hour)/6076(ft in nautical mile)
0.75*5500/2*60/6076=20.36kn
The same calc for 5” gives11.31kn

 

[C08]

Thanks - running my prop through that calculation with 2.42 gearbox reduction gives 9.35 kn and with 20% slip around 7.5Kn which sounds about what I would expect.. Just need to find a prop without straining my budget even further.

 

[B27]

Thanks - running my prop through that calculation with 2.42 gearbox reduction gives 9.35 kn and with 20% slip around 7.5Kn which sounds about what I would expect.. Just need to find a prop without straining my budget even further.

Slip will probably be a lot more than 20% unless blade area is high.

If the engine won't rev even when the boat has built up speed, it's a long way wrong IMHO.
A smaller diameter prop might be worth a try.

I recommend Dave Gerr's book on props.

Also check the gear ratio, and maybe consider borrowing a small boat to test the engine with?
Is the hull clean?

 

[Boathook]

I've got a Yamaha high thrust prop (10 5/8" by 8 1/4") machined to fit my Honda 30 on my 9m Catalac. Allows the engine to reach over 5000 rpm and gives a top speed of around 6.6 knots. Gives better reverse than a Honda prop of the same pitch. I cruise under power with the engine between 3000 and 4000 rpm depending upon sea state.

This prop Yamaha 10 5/8" x 8 1/4" 20/25/30hp Genuine High Thrust Outboard Propeller Type F | eBay

 

[C08]

Slip will probably be a lot more than 20% unless blade area is high.

If the engine won't rev even when the boat has built up speed, it's a long way wrong IMHO.
A smaller diameter prop might be worth a try.

I recommend Dave Gerr's book on props.

Also check the gear ratio, and maybe consider borrowing a small boat to test the engine with?
Is the hull clean?

Thanks for the suggestion but weighing about 100kg that is not an option

 

[Boathook]

Thanks for the suggestion but weighing about 100kg that is not an option

What outboard do you have ?

 

[Chiara’s slave]

My prop is a Polastorm 4 blade high thrust job. High blade area, with zero camber, as reverse is just as important as fwd to us. Not speed of course, but stopping and steering a high windage boat.

 

[B27]

Thanks for the suggestion but weighing about 100kg that is not an option

I guess that's about 75HP of four stroke?
So you're likely not asking in the best forum.
Our experience of displacement sailing yachts is not ideal, not sure what I've learned from RIBs is exactly relevant either, except that those extra strokes weigh a lot and 4 strokes can have an apparent shortage of mid-range torque compared to a 2-stroke of similar weight.
Boats which use such big outboards to do under ten knots are not exactly commonplace.

What's the boat?

I think my suggestion of Gerr still holds.

 

[Boathook]

I guess that's about 75HP of four stroke?
So you're likely not asking in the best forum.
Our experience of displacement sailing yachts is not ideal, not sure what I've learned from RIBs is exactly relevant either, except that those extra strokes weigh a lot and 4 strokes can have an apparent shortage of mid-range torque compared to a 2-stroke of similar weight.
Boats which use such big outboards to do under ten knots are not exactly commonplace.

What's the boat?

I think my suggestion of Gerr still holds.

@C08 He has or had a 8m Catalac, possibly around 4 tons weight. I thought he mentioned the engine in a post but having looked he didn't. Popular outboards for this boat have been a high thrust, mainly yamaha 9,.9 to 25 hp. The 25hp is around 100kg, the extra weight being in the mounting bracket and gearbox.

 

[B27]

I tend to err in the direction of wanting lots of thrust into a headwind or should I ever run aground etc, so achieving all the revs you ever want in flat water should be a given. Aiming for too high a cruising speed isn't good.
Ideally you need the torque/rpm graph of the engine. If you know the power at the revs it's doing you have atl east some sort of data point.

The online prop calculators may have too low a blade area % for outboard props?

Weighing off diameter against pitch can be important.
The Yamaha data Google throws at suggests a gear ratio of 2.08?

 

[Chiara’s slave]

I tend to err in the direction of wanting lots of thrust into a headwind or should I ever run aground etc, so achieving all the revs you ever want in flat water should be a given. Aiming for too high a cruising speed isn't good.
Ideally you need the torque/rpm graph of the engine. If you know the power at the revs it's doing you have atl east some sort of data point.

The online prop calculators may have too low a blade area % for outboard props?

Weighing off diameter against pitch can be important.
The Yamaha data Google throws at suggests a gear ratio of 2.08?

You’d think the largest reasonable diameter and lots of blade area would be the way to go. As I say, it’s worked for us, we are large, heavy and high windage for the engine power. We likewise erred on the side of headwinds and chop rather than the highest possible speed. But then, we hardly ever motor, just in and out of the harbour. We don’t give a stuff about fuel consumption or cruising speed, we only need 6-8kn of wind to comfortably outperform the engine with sail power. It’s all a compromise for the type of boat, and the use. We might have made a slightly different choice if we’d had miles to motor to reach open water.

 

[B27]

You’d think the largest reasonable diameter and lots of blade area would be the way to go. As I say, it’s worked for us, we are large, heavy and high windage for the engine power. We likewise erred on the side of headwinds and chop rather than the highest possible speed. ....

If you go for a large blade area, then it takes a lot of power to run at high % of slip.
This can mean the engine won't turn the prop quick enough when the boat is going slowly or into waves etc.
More so with a high revving engine which has mediocre torque at lower rpm.
We see this with our club RIB, if we overload it, the boat struggles to get on the plane because you can't get full power from the engine which refuses to hit full revs until you're on the plane. Whereas e.g. the little outboard for my tender will rev out in a bollard pull test and yet do 10 knots planing a small light dinghy.
If you have a big prop and pitch it for bollard pull tests, you will get a poor top speed and the engine will be knocked into overload every time you hit a wave.
If you have a small prop it works more by throwing water backwards and the exchange of momentum is a force which pushes the boat forwards, like a jet engine. Effective and flexible but not so efficient at low speeds.
The big prop is a tight coupling between the boat and the water, the small prop a looser coupling.
The optimum prop compromises, to allow the engine to operate favourably.
It's about moving around the torque/slip graphs as the load changes.

It can be a big subtle subject, that's why people have written books about it.

It's a lot easier if your starting point is ample power for the speed you aspire to. And a diesel engine with yards of torque from 1200 rpm upwards.